Lithographic printing plates have been widely used for many years. One of the basic concepts utilized in that technology is the establishment of differential wettability between areas on the planographic printing surface. This is most usually effected by providing a substrate having a hydrophilic or water wettable surface and coating that surface with an imageable and developable (usually light or radiation sensitive) grease sensitive (oleophilic) and hydrophobic layer. After imaging and developing of this layer, the printing plate variously exposes hydrophilic and hydrophobic surfaces. When the plate is then wet with water and then coated with grease or oil-based printing inks, only the hydrophobic areas of the image will hold the ink and provide a dark image when pressed against a receiving surface such as paper.
This lithographic process has been able to provide planographic printing plates which are capable of producing as few as hundreds of copies or as many as several hundred thousand high quality copies. The desirability of being able to produce large numbers of copies from a single plate are quite apparent. Only a single imaging and developing procedure must be performed, and the printing press need not be shut down during operation in order to change plates.
The investigation of means for lengthening the running time of planographic printing plates has been the focus of many studies and research. It is also necessary in the provision of more durable plates to maintain or improve such other desirable or essential characteristics of printing plates such as imaging speed, ease of development, non-polluting chemistry, and shelf life. Most of the research in providing longer running and higher durability planographic printing plates has centered on the imageable and hydrophobic coating layer on the hydrophilic surface of the substrate. As it is usually the breakdown of this layer which causes failure of the printing plate, the logic of that direction of research is apparent.
In general, the substrate provided for the imageable layer is an aluminum sheet which has been prepared for coating by a variety of cleaning, mechanical, and chemical treatments. For example, an aluminum surface is usually cleaned to remove oils and other contaminants. This cleaning is then followed by a mechanical or electrochemical graining process, and finally an anodizing process.
For example, U.S. Pat. No. 3,963,594 discloses the electrochemical etching of an aluminum substrate with an aqueous solution of hydrochloric acid and gluconic acid. This provides a uniformly coarse surface texture to the aluminum substrate. The process of that patent permits the use of lower current densities than were found to be necessary when using only hydrochloric acid in the bath. The treatment also complexed dissolved aluminum and other impurities which form in the etching bath. This extended the life of the bath. Desmutting treatments were also shown in combination with the electrochemical etching.
British Pat. No. 1,439,127 discloses an anodizing treatment for aluminum substrates in which the anodization is performed in an aqueous sulfuric acid bath at a temperature in excess of 70.degree. C. and with an anodizing current density of at least 50 amps/sq. ft. This treatment reduces the length of time in which the anodizing step is performed. Other etching solution compositions and electrical parameters are disclosed in U.S. Pat. Nos. 4,072,589 and 4,052,275 and French Pat. No. 2,322,015.
U.S. Pat. No. 3,030,210 and French Pat. No. 2,025,550 disclose lithographic plate substrates prepared by immersion in phosphate-containing solutions, rinsing, and drying. The phosphate-containing layer produced by such a method is comparatively thin, soft and hydrated and does not have the intrinsic wear-resisting properties conferred by firing a ceramic coating at elevated temperatures.
After electrochemical or mechanical etching of the aluminum surface, an anodically oxidizing treatment is usually performed to render the aluminum surface both corrosion resistant and wear resistant. This is shown in British Pat. No. 1,439,127 discussed above and in U.S. Pat. No. 4,131,518. In this latter patent, aluminum foil in the form of a continuous web, particularly when the aluminum carries a polymeric coating on one side thereof, is anodized so that energy requirements are reduced and anodizing speeds are increased.
U.S. Pat. No. 3,181,461 discloses an anodizing process in which the sulfuric acid anodizing step is followed by treatment with an aqueous solution of sodium silicate. This treatment seals the pores of the anodic oxide and provides a hydrophilic, ink-repelling surface layer.
The practice of treating lithographic substrates of aluminum in order to increase surface areas and enable anodic coatings is well described in the art such as U.S. Pat. Nos. 3,935,080; 3,929,591; 3,980,539; and 3,988,217.
Even though anodizing has become the most common means of providing an aluminum substrate for durable planographic printing plates, and even though some reduction of energy requirements has been made, the process still requires large amounts of electrical energy in its operation and also generates effluents that must be carefully disposed of to avoid environmental pollution.
A novel method for providing textured surfaces on lithographic printing plate substrates of aluminum was disclosed in U.S. Pat. No. 3,210,184. A layer of boehmite (aluminum oxide monohydrate) was produced on the aluminum substrate by bathing the aluminum in hot water or steam in the presence of a weak organic base. Printing plates using this textured substrate were shown to provide increased numbers of copies under comparable conditions as compared to printing plates using mechanically roughened aluminum substrates.
German Pat. (Offenlegungsschrift) No. 24 34 098 discusses firing a composition of aluminum phosphate and silicon carbide particles onto a metal surface for the purpose of increasing its wear resistance. However, the invention of German Pat. No. 24 34 098 is not suitable for use on lithographic plates and is not performed on aluminum surfaces.
U.S. Pat. Nos. 3,871,881 and 3,975,197 disclose another method of enhancing the physical properties of aluminum surfaces. Various types of particulate material are bonded to the surface of the aluminum by an in situ formed binder of aluminum hydroxyoxide. The enhanced aluminum article is suggested for use as a substrate for printing plates.
It is disclosed in the description of the present invention that a novel process for treating lithographic quality aluminum foil can provide a durable, long-running substrate for lithographic plate constructions. This process provides a novel substrate which can be resistant to the chemical action of printing plate developers and press solvents. The novel substrate also enables the bonding of many photoreactive imaging layers to the treated aluminum substrate without the need for primers or other adhesion promoting agents.